1. Field of the Invention
The present invention relates to a cerium-containing abrasive suitable for precisely polishing a glass sheet or a semiconductor substrate.
2. Discussion of Background
Heretofore, a cerium type abrasive has been widely used for polishing the surface of glass such as glass for lenses, optical glass, sheet glass, bulb glass, a glass substrate for magnetic disk, glass for photomask, a glass substrate for TFT, and it is known to exhibit a very high polishing rate.
Further, a cerium type abrasive has recently been used not only for glass but also as an abrasive (an abrasive slurry) in a chemical mechanical polishing (hereinafter referred to as CMP) method which is a polishing technique for ultraprecise planarization of an insulating layer and/or a metal layer formed on a semiconductor substrate in the process for producing a LSI device having a multilayered wiring structure.
However, in an application where an extremely high level of flatness is required like precise polishing for mirror finish of a glass surface or CMP of a semiconductor substrate, it is difficult to realize a sufficient polishing rate and a high level of flatness solely by means of a conventional cerium type abrasive.
Accordingly, it has been proposed to carry out a plurality of polishing steps in combination with a polishing step by means of a colloidal silica abrasive, but there has been a problem such that the process steps and installation tend to be complex, as extra installations for polishing steps will be required, and the preparation time for switching will be wasted.
Further, in the case of a glass sheet produced by a so-called float process wherein molten glass is permitted to continuously flow and advance while floating on a molten metal bath surface, it has been found by our study that on such a glass surface, a layer of the metal of the metal bath, such as tin or a tin compound, is formed, and it is not easy to sufficiently remove this metal layer solely by means of a conventional cerium type abrasive.
Accordingly, it is desired to develop an abrasive which has a higher ability to polish a glass surface and which is suitable also for planarization of a semiconductor substrate.
It is an object of the present invention to provide an abrasive having both a high polishing rate and an excellent planarization ability in polishing a glass surface and in planarization of a semiconductor substrate.
(1) The present invention provides an abrasive comprising at least the following components (A) and (B):
(A) cerium-containing abrasive grains having an average particle size of from 0.5 to 5.0 xcexcm; and
(B) particles having an average particle size of from 0.01 to 0.3 xcexcm, which are particles of at least one member selected from the group consisting of aluminum oxide, silicon dioxide, zirconium oxide, titanium oxide, silicon nitride and manganese oxide.
(2) Further, the present invention provides a polishing method for a glass surface, which comprises polishing the glass surface with the above abrasive.
(3) Furthermore, the present invention provides a planarization method for a semiconductor substrate, which comprises polishing an insulating layer and/or a metal layer formed on a semiconductor substrate, with the above-mentioned abrasive.
Further embodiments of the present invention will be made apparent from the following description.
The abrasive of the present invention comprises at least the following components (A) and (B):
(A) cerium-containing abrasive grains having an average particle size of from 0.5 to 5.0 xcexcm (hereinafter referred to also as component (A)); and
(B) particles of at least one member selected from the group consisting of aluminum oxide, silicon dioxide, zirconium oxide, titanium oxide, silicon nitride and manganese oxide, which has an average particle size of from 0.01 to 0.3 xcexcm (hereinafter referred to also as component (B)).
Here, the cerium-containing abrasive grains are abrasive grains comprising cerium oxide and other rare earth oxide as the main components, which are so-called a cerium type abrasive to be used for polishing e.g. glass. Such abrasive grains are obtainable mainly from a rare earth ore such as a rare earth carbonate via a step of e.g. calcination.
The average particle size of the cerium-containing abrasive grains (component (A)) in the present invention is from 0.5 to 5.0 xcexcm in order to satisfy both the high polishing power and reduction of scratching. If the particle size is substantially larger than this range, scratching will increase, and if it is substantially smaller, the polishing power will deteriorate. The cerium-containing abrasive grains preferably contain cerium oxide (ceria, CeO2) in an amount of at least 50 mass %, preferably at least 60 mass %, more preferably at least 80 mass %, from the viewpoint of the polishing power and easiness in its production. Especially when the abrasive is to be used for planarization of a semiconductor substrate, cerium oxide is preferably at least 98 mass %, more preferably at least 99.8 mass %, since it is difficult to remove any remaining impurities after polishing. The cerium-containing abrasive grains may be in the form of a powder or a sol. However, from the viewpoint of the handling efficiency, the powder form is preferred.
In the present invention, for the average particle size, a particle size distribution on the basis of mass is obtained to prepare a cumulative curve for a total mass of 100%, and a particle size at a point where the cumulative curve becomes 50%, is taken as the average particle size. This average particle size may be referred to also as a mass basis cumulative 50% particle size (see e.g. Chemical Engineering Handbook, 5th edition, compiled by Chemical Engineering Association, p. 220-221).
The measurement of such an average particle size is carried out, for example, by using an instrument such as Microtrack HRAX-100, manufactured by Nikkiso K.K., in such a manner that abrasive grains are subjected to ultrasonic wave treatment in a medium such as water, and the particle size distribution is measured when the dispersion state of the abrasive grains has been stabilized.
The particle size of cerium oxide can be adjusted also by the calcination temperature when the low material such as a rare earth carbonate is calcined in an electric furnace. Usually, the higher the calcination temperature, the larger the particle size. Accordingly, a suitable calcination temperature is selected. Further, it is also possible to selectively obtain only particles having a certain particle size by e.g. classification after the calcination.
The particles of component (B) in the abrasive of the present invention are characterized in that they are basically hard particles having a hardness higher than component (A) and having a particle size substantially smaller than component (A). Specifically, their average particle size is usually from 0.01 to 0.3 xcexcm in order to reduce scratching. Particularly for polishing a glass surface, the average particle size is preferably from 0.1 to 0.3 xcexcm. If the particle size is too small, no adequate effect of addition of component (B) tends to be obtained, and if the particle size is too large, the hardness tends to be high, and scratching is likely to occur.
The type of the particles constituting component (B) is suitably selected depending upon the object to be polished. However, from the viewpoint of the cost and the polishing power, it is preferably selected from aluminum oxide (alumina, A12O3), silicon dioxide (silica, SiO2), zirconium oxide (zirconia, ZrO2), titanium oxide (titania, TiO2), silicon nitride (Si3N4) and manganese oxide (MnO2, Mn2O3, Mn3O4, etc). Among them, alumina, zirconia or silica is particularly preferred.
For polishing a glass surface, A12O3 is particularly preferred from the viewpoint of the polishing power. To avoid scratching, it is particularly preferred to suitably select a-alumina having a higher hardness or xcex3-alumina having a fine particle size. As alumina particles, those having natures represented by the following (1) and (2) and their mixtures are, for example, more preferred from the viewpoint of easy production or availability:
(1) Alumina particles having an average particle size of from 0.1 to 0.3 xcexcm and a crystal form of xcex1-alumina; and
(2) Alumina particles having an average particle size of from 0.01 to 0.15 xcexcm and having a crystal form of xcex3alumina.
In the abrasive of the present invention, the mass ratio of component (A) to component (B) is preferably from 1:0.001 to 1:0.3, more preferably from 1:0.005 to 1:0.2, from the viewpoint of the polishing power.
The method of use of the abrasive of the present invention is not particularly limited. However, it is advisable to use it in the form of a slurry having a solid content of from 0.1 t 30 mass %, preferably from 1 to 15 mass % (hereinafter referred to also as an abrasive slurry) by thoroughly dispersing it in water by an agitation mixing machine or a homogenizer. Further, to such a slurry, an additive such as a dispersant, a thickener, an oxidizing agent or a pH-regulating agent, may be suitably incorporated, as the case requires.
In such a case, it is also possible to prepare an abrasive slurry from a powder of CeO2 as component (A) and a dispersion having component (B) such as alumina dispersed in a medium.
Polishing by means of such an abrasive slurry, can be carried out in accordance with a conventional method. For example, polishing of a glass surface or planarization of a semiconductor substrate may be carried out while supplying the abrasive slurry to a polishing section in an apparatus comprising a polishing head located above and provided with a driving means to hold and rotate an object to be polished such as a glass sheet or a semiconductor substrate, and a rotational platen located below to face the polishing head and having a polishing pad attached thereto.
The abrasive of the present invention is useful for polishing a glass surface and is used for e.g. glass for lenses, optical glass, sheet glass, bulb glass, glass magnetic disk, glass for photomask, or a glass substrate for TFT. The abrasive of the present invention has an adequate polishing power especially against a glass prepared by a float process and having a coating layer or blocks of e.g. tin oxide attached on the surface.
Further, the abrasive of the present invention is useful also for a planarization method for a semiconductor substrate which comprises polishing an insulating layer and/or a metal layer formed on a semiconductor substrate. When the abrasive is applied to CMP, the polishing powers against the insulating layer and the metal layer may, respectively, be adjusted to be optimum by suitably selecting the type, particle size and content of component (B).
The abrasive of the present invention comprising at least component (A) and component (B), has a high polishing power which can not be obtained by cerium alone, as will be shown in the Examples given hereinafter, and it can effectively be used for polishing a glass on which tin oxide or the like remains, without bringing about scratching.
At present, the detailed polishing mechanism by the abrasive of the present invention is not clearly understood, but is considered to be as follows. Namely, with the conventional cerium-containing abrasive grains (component (A)) only, the polishing power is weak or the planarization ability is low. Whereas, when a small amount of particles (component (B)) having a particle size smaller and a hardness higher than the cerium particles and having a different type of polishing action, are incorporated to such cerium-containing abrasive grains, the two components act on the surface to be polished to provide the respective merits during the polishing process, whereby the polishing power and the planarization ability which can not be obtained by the cerium-containing abrasive grains (component (A)) only, can be complemented by component (B), and the required performance can be attained by a single (one step) polishing operation.
Further, it is considered that by properly selecting component (B) for its type, particle size and crystal form and incorporating it in a specific amount to component (A), it is possible to maintain a high polishing power against any glass or semiconductor substrate to be polished, without bringing about scratching.
Thus, a plurality of polishing steps which used to be necessary since the polishing power was inadequate in the prior art, will be unnecessary. The present invention thus provides substantial merits in view of the cost, the operation efficiency and the installation.
Further, a glass produced by a float process has a tin or tin compound layer, such as tin oxide, formed on its surface, and it has been difficult to remove such a layer by a conventional cerium type abrasive only. Whereas, as mentioned above, component (B) such as alumina in the abrasive of the present invention has a proper polishing power against such a layer, whereby it is possible to remove the layer. Thus, the abrasive of the present invention is particularly useful for polishing the surface of a glass produced by a float process.